Schottky principal G-bundles over compact Riemann surfaces

Fundação para a Ciência e a Tecnologia (FCT)- PhD grant SFRH/BD/37151/2007; projects PTDC/MAT/099275/2008; PTDC/MAT/119689/2010; PTDC/MAT/120411/2010; PTDC/MAT-GEO/0675/2012

Mechanical characterization of aortas using 2D ultrasound elastography

Rupture of aortic aneurysms (AA) is a major cause of death in the Western world. Currently, clinical decision upon surgical intervention is based on the diameter of the aneurysm. However, this method is not fully adequate. Noninvasive assessment of the elastic properties of the arterial wall can be a better predictor for AA growth and rupture risk. The purpose of this study is to estimate mechanical properties of the aortic wall using in vitro inflation testing and 2D ultrasound (US) elastography, and investigate the performance of the proposed methodology for physiological conditions. Two different inflation experiments were performed on twelve porcine aortas: 1) a static experiment for a large pressure range (0 – 140 mmHg); 2) a dynamic experiment closely mimicking the in vivo hemodynamics at physiological pressures (70 – 130 mmHg). 2D raw radiofrequency (RF) US datasets were acquired for one longitudinal and two cross-sectional imaging planes, for both experiments. The RF-data were manually segmented and a 2D vessel wall displacement tracking algorithm was applied to obtain the aortic diameter–time behavior. The shear modulus G was estimated assuming a Neo-Hookean material model. In addition, an incremental study based on the static data was performed to: 1) investigate the changes in G for increasing mean arterial pressure (MAP), for a certain pressure difference (30, 40, 50 and 60 mmHg); 2) compare the results with those from the dynamic experiment, for the same pressure range. The resulting shear modulus G was 94 ± 16 kPa for the static experiment, which is in agreement with literature. A linear dependency on MAP was found for G, yet the effect of the pressure difference was negligible. The dynamic data revealed a G of 250 ± 20 kPa. For the same pressure range, the incremental shear modulus (Ginc) was 240 ± 39 kPa, which is in agreement with the former. In general, for all experiments, no significant differences in the values of G were found between different image planes. This study shows that 2D US elastography of aortas during inflation testing is feasible under controlled and physiological circumstances. In future studies, the in vivo, dynamic experiment should be repeated for a range of MAPs and pathological vessels should be examined. Furthermore, the use of more complex material models needs to be considered to describe the non-linear behavior of the vascular tissue.

Soft biomaterials as a substitute for the Nucleus Pulposus

One of the largest health problems faced worldwide, when evaluated by direct (clinical) as well indirect cost (absenteeism), is the degeneration of the intervertebral disc (IVD) that leads to back pain and, potentially disability and individual´s quality of life decreasing. The intervertebral disc is a mechanical and biological complex structure, formed by a tough outer layer of fibrocartilage called Annulus Fibrosus (AF),which surrounds a soft, elastic and gelatinous core called Nucleus Pulposus (NP). These two structures are completed by two upper and lower encasing layer called Vertebral Endplates (VEP). The degeneration of the IVD is marked by the dehydration of the Nucleus Pulposus, reducing the hydrostatic pressure inside the nucleus, resulting in a loss of capability to support compressive forces, during the active period, and to regain height during the resting period. This situation will compromise the role of shock absorber by the NP and transfers these forces to the AF. This transfer will result in cracks on the AF, deteriorating the IVD, allowing the ingrowth of vessels and nerves. This project was based on the developing a protocol to test suitable NP replacements, in hope to future assessment of discrete mechanical values and characteristics for an NP replacement. For this, Nucleus pulposus samples from goat, encapsulated Hydromed gel denominated “Raviolis” and Chitosan gels, produced via wet route using an ammonium environment, were confined compressed. Chitosan was rheologically tested and swelling capability of all the three type of materials was assessed. Results showed that the Nucleus Pulposus and “Raviolis” have similar mechanical behavior, being able to swell and “build up” hydrostatic pressure after a compression stage, while the Chitosan gel did not showed that ability. Therefore, “Raviolis” are a more suitable candidate to replace the NP than Chitosan gels. It was also observed that confined compression is the key test to perform on any possible candidate to replace the NP.

Proprioceptive neuromuscular facilitation in HTLV-I-associated myelopathy/tropical spastic paraparesis

Introduction: Human T cell lymphotropic virus type I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) can impact the independence and motricity of patients. The aims of this study were to estimate the effects of physiotherapy on the functionality of patients with HAM/TSP during the stable phase of the disease using proprioceptive neuromuscular facilitation (PNF) and to compare two methods of treatment delivery. Methods: Fourteen patients with human T cell lymphotropic virus type I (HTLV-I) were randomly allocated into two groups. In group I (seven patients), PNF was applied by the therapist, facilitating the functional activities of rolling, sitting and standing, walking and climbing and descending stairs. In group II (seven patients), PNF was self-administered using an elastic tube, and the same activities were facilitated. Experiments were conducted for 1h twice per week for 12 weeks. Low-back pain, a modified Ashworth scale, the functional independence measure (FIM) and the timed up and go test (TUG) were assessed before and after the interventions. Results: In the within-group evaluation, low-back pain was significantly reduced in both groups, the FIM improved in group II, and the results of the TUG improved in group I. In the inter-group analysis, only the tone was lower in group II than in group I. Conclusions: Both PNF protocols were effective in treating patients with HAM/TSP.

Prenatal tracheal ligation or intra-amniotic administration of surfactant or dexamethasone prevents some structural changes in the pulmonary arteries of surgically created diaphragmatic hernia in rabbits

PURPOSE: Characterization of the structural changes occurring in the pulmonary arteries resulting from surgically produced congenital diaphragmatic hernia in rabbits, with particular emphasis on the preventive effects of prenatal tracheal ligation or administration of intra-amniotic dexamethasone or surfactant. METHODS: Twenty rabbit fetuses underwent surgical creation of a left-sided congenital diaphragmatic hernia on the 24th or 25th gestational day. They were divided according to the following procedures: congenital diaphragmatic hernia (n = 5), congenital diaphragmatic hernia plus tracheal ligation (n = 5), congenital diaphragmatic hernia plus intra-amniotic administration of dexamethasone 0.4 mg (n = 5) or surfactant (Curosurf 40 mg, n = 5). On gestational day 30, all the fetuses were delivered by caesarean section and killed. A control group consisted of five nonoperated fetuses. Histomorphometric analysis of medial thickness, cell nuclei density, and elastic fiber density of pulmonary arterial walls was performed. RESULTS: Arteries with an external diameter > 100 mum have a decreased medial thickness, lower cell nuclei density, and greater elastic fiber density when compared with arteries with external diameter <= 100 mum. Congenital diaphragmatic hernia promoted a significant decrease in medial thickness and an increase in cell nuclei density in artery walls with external diameter > 100 mum. Prenatal treatments with tracheal ligation or intra-amniotic administration of dexamethasone or surfactant prevented these changes. In arteries with external diameter <= 100 mum, congenital diaphragmatic hernia promoted a significant increase in medial thickness and in cell nuclei density and a decrease in elastic fiber density. The prenatal treatments with tracheal ligation or intra-amniotic administration of dexamethasone or surfactant prevented these changes, although no effect was observed in elastic fiber density in the congenital diaphragmatic hernia plus dexamethasone group. CONCLUSIONS: Congenital diaphragmatic hernia promoted different structural changes for large or small arteries. The prenatal intra-amniotic administration of dexamethasone or surfactant had positive effects on the lung structural changes promoted by congenital diaphragmatic hernia, and these effects were comparable to the changes induced by tracheal ligation.

Compact saloplastic membranes of natural polysaccharides for soft tissue engineering

The regeneration of soft biological tissues requires new substitutes that exhibit mechanical properties similar to the native tissue. Herein, thin saloplastic membranes with tunable physical properties are prepared by complexation of chitosan and alginate solutions containing different concentrations of sodium chloride. Polyelectrolyte complexes (PECs) are transferred to flat Petri dishes for compaction into membrane shapes by sedimentation and solvent evaporation. All membranes are resistant to degradation by lysozyme and are stable in solutions with pH values between 1 and 13. Immersing the different membranes in new doping solutions of increasing salt concentrations triggers the typical saloplastic behavior, with a high water absorption and decrease of the rigidity and ultimate tensile strength. The range of such variations is tuned by the sodium chloride amount used in the synthesis: high salt concentrations increase water uptake and tensile moduli, while decreasing the ultimate strength. Cellular assays demonstrate high proliferation rates and viability of L929 fibroblasts seeded onto the most rigid membranes. The results validate the use of saloplastic membranes as soft tissue substitutes for future biomedical applications.

Monitoring the early stiffness development in epoxy adhesives for structural strengthening

The present work aimed to assess the early-age evolution of E-modulus of epoxy adhesives used for Fibre-Reinforced Polymer (FRP) strengthening applications. The study involved adapting an existing technique devised for continuous monitoring of concrete stiffness since casting, called EMM-ARM (Elasticity Modulus Measurement through Ambient Response Method) for evaluation of epoxy stiffness. Furthermore, monotonic tensile tests according to ISO standards and cyclic tensile tests were carried out at several ages. A comparison between the obtained results was performed in order to better understand the performance of the several techniques in the assessment of stiffness of epoxy resins. When compared to the other methodologies, the method for calculation of E-modulus recommended by ISO standard led to lower values, since in the considered strain interval, the adhesive had a non-linear stress–strain relationship. The EMM-ARM technique revealed its capability in clearly identifying the hardening kinetics of epoxy adhesives, measuring the material stiffness growth during the entire curing period. At very early ages the values of Young׳s modulus obtained with quasi-static tests were lower than the values collected by EMM-ARM, due to the fact that epoxy resin exhibited a significant visco-elastic behaviour.

Mechanical characterization of old chestnut beams

The main objective of this work is to evaluate, by non-destructive techniques, seven old Chestnut beams. For that, after the geometric assessment and the detailed visual inspection that allowed to strength grade the beams, a series of non-destructive tests was setup. In a first step, non-destructive bending tests, under the elastic limit, were performed to quantify the modulus of elasticity in bending (MoE) of the seven beams. Then, Resistograph® and Pilodyn® tests were done to assess the superficial decay and to have aclearer idea of the voids dimensions. Then, two beams were tested in bending until failure to evaluate the bending strength. In a second step, end parts were cut from the beams, one per end of the beams, to perform Resistograph®, Pilodyn® and ultrasound tests, to quantify the density of the beams and to extract meso-specimens to be used in tension parallel to the grain tests

Effect of binder on performance of intumescent coatings

This study investigates the role of the polymeric binder on the properties and performance of an intumescent coating. Waterborne resins of different types (vinylic, acrylic, and styrene-acrylic) were incorporated in an intumescent paint formulation, and characterized extensively in terms of thermal degradation behavior, intumescence thickness, and thermal insulation. Thermal microscopy images of charred foam development provided further information on the particular performance of each type of coating upon heating. The best foam expansion and heat protection results were obtained with the vinyl binders. Rheological measurements showed a complex evolution of the viscoelastic characteristics of the materials with temperature. As an example, the vinyl binders unexpectedly hardened significantly after thermal degradation. The values of storage moduli obtained at the onset of foam blowing (melamine decomposition) were used to explain different intumescence expansion behaviors.

Creep behaviour of cracked steel fibre reinforced self-compacting concrete laminar structures

Tese de Doutoramento - Civil Engineering

High performance electromechanical actuators based on ionic liquid/poly(vinylidene fluoride)

Due to the increasing need of low voltage actuators, independent from electrochemical processes, electroactive actuators based on poly(vinylidene fluoride) composites with 10, 25 and 40 % of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2mim] [NTf2], ionic liquid are prepared by solvent casting and melting. We show that the charge structure of [C2mim] [NTf2] induces the complete piezoelectric -phase crystallization of the PVDF within the composite and decreases its crystallinity fraction significantly. [C2mim] [NTf2] also works as a plasticizer of PVDF, reducing the elastic modulus down to 12 % of the initial value. Moreover, the composites show significant displacement and bending under applied voltages of 2, 5 and 10 Vpp. The displacement and bending of the composite membranes are also evaluated as a function of [C2mim] [NTf2] content and sample thickness. Increasing amounts of ionic liquid result in larger deformations independently of the applied voltage.

Effect of ionic liquid anion and cation on the physico-chemical properties of poly(vinylidene fluoride)/ionic liquid blends

Poly(vinylidene fluoride), PVDF, has been blended with different ionic liquids (IL) in order to evaluate the effect of the different IL anions and cations on the electroative -phase, thermal, mechanical and electrical properties of the polymer blend. [C2MIM][Cl], [C6MIM][Cl], [C10MIM][Cl], [C2MIM][NTf2], [C6MIM][NTf2], [C10MIM][NTf2] have been selected and were introduced in the polymer at a weight percentage of 40 wt%. It was found that the incorporation of ILs into the PVDF matrix leads to an increase of the -phase content due to the strong electrostatic interactions between the dipolar moments of PVDF and the ILs. Further, the incorporation of ILs into PVDF strongly decreases the elastic modulus and increases the electrical conductivity of the blend with respect to the pure polymer matrix, all these effects being accompanied by a modification of the crystallization kinetics, as indicated by the modified spherulitic microstructure. Thus, novel PVDF/IL blends films with high transparency, excellent antistatic properties, and highly polar crystal form fraction were successfully achieved.

Effect of the degree of porosity on the performance of poly(vinylidene fluoride-trifluoroethylene)/poly(ethylene oxide) blend membranes for lithium-ion battery separators

Porous polymer membranes based on poly(vinylidene fluoride-trifluoroethylene)/poly(ethylene oxide) copolymers, P(VDF-TrFE)/PEO, are prepared through the, from partial to total, elimination of PEO, leading to interconnected micropores in the polymer blends. Electrolyte uptake, thermal and mechanical properties depend on the amount of PEO present in the polymer blend. Further, the degree of crystallinity of PEO and the elastic modulus (E´) of the polymer blend decrease with increasing PEO removal. Electrical properties of the polymer blend membranes are influenced by the porosity and are dominated by diffusion. The temperature dependence of ionic conductivity follows the Arrhenius behavior. It is the highest for the membranes with a volume fraction of pores of 44% (i.e, 90% PEO removal), reaching a value of 0.54 mS.cm-1 at room temperature. Battery performance was determined by assembling Li/C-LiFePO4 swagelok cells. The polymer blends with 90% PEO removal exhibit rate (124 mAhg-1 at C/5 and 47 mAhg-1 at 2C) and cycling capabilities suitable for lithium ion battery applications.

Mechanical vs. electrical hysteresis of carbon nanotube/styrene-butadiene-styrene composites and their influence in the electromechanical response

The interesting properties of thermoplastics elastomers can be combined with carbon nanotubes (CNT) for the development of large strain piezoresistive composites for sensor applications. Piezoresistive properties of the composites depend on CNT content, with the gauge factor increasing for concentrations around the percolation threshold, mechanical and electrical hysteresis. The SBS copolymer composition (butadiene/styrene ratio) influences the mechanical and electrical hysteresis of composites and, therefore, the piezoresistive response. This work reports on the electrical and mechanical response of CNT/SBS composites with 4%wt nanofiller content, due to the larger electromechanical response. C401 and C540 SBS copolymers with 80% and 60% butadiene content, respectively have been selected. The copolymer with larger amount of soft phase (C401) shows a rubber-like mechanical behavior, with mechanical hysteresis increasing linearly with strain until 100% strain. The copolymer with the larger amount of hard phase (C540) just shows rubber-like behavior for low strains. The piezoresistive sensibility is similar for both composites for low strains, with a GF≈ 5 for 5% strain. The electrical hysteresis shows opposite behavior than the mechanical hysteresis, increasing with strain for both composites, but with higher increase for softer copolymer, C401. The GF increases with increasing strain, but this increase is larger for composites with lower amounts of soft phase due to the distinct initial modulus and deformation of the soft and hard phases of the copolymer. The soft phase shows larger strain under a given stress than the harder phase and the conductive pathway rearrangements in the composites are different for both phases, the harder copolymer (C540) showing higher piezoresistive sensibility, GF≈ 18, for 20% strain.

Mechanical fatigue performance of PCL-chondroprogenitor constructs after cell culture under bioreactor mechanical stimulus

In tissue engineering of cartilage, polymeric scaffolds are implanted in the damaged tissue and subjected to repeated compression loading cycles. The possibility of failure due to mechanical fatigue has not been properly addressed in these scaffolds. Nevertheless, the macroporous scaffold is susceptible to failure after repeated loading-unloading cycles. This is related to inherent discontinuities in the material due to the micropore structure of the macro-pore walls that act as stress concentration points. In this work, chondrogenic precursor cells have been seeded in Poly-ε-caprolactone (PCL) scaffolds with fibrin and some were submitted to free swelling culture and others to cyclic loading in a bioreactor. After cell culture, all the samples were analyzed for fatigue behavior under repeated loading-unloading cycles. Moreover, some components of the extracellular matrix (ECM) were identified. No differences were observed between samples undergoing free swelling or bioreactor loading conditions, neither respect to matrix components nor to mechanical performance to fatigue. The ECM did not achieve the desired preponderance of collagen type II over collagen type I which is considered the main characteristic of hyaline cartilage ECM. However, prediction in PCL with ECM constructs was possible up to 600 cycles, an enhanced performance when compared to previous works. PCL after cell culture presents an improved fatigue resistance, despite the fact that the measured elastic modulus at the first cycle was similar to PCL with poly(vinyl alcohol) samples. This finding suggests that fatigue analysis in tissue engineering constructs can provide additional information missed with traditional mechanical measurements.